1. Field of the Invention
This invention relates generally to refrigeration systems and more particularly, it relates to capacity control systems for inverter-driven centrifugal compressor based water chillers wherein adjustable inlet guide vanes and compressor speed are both regulated in response to a measurement of suction flow to realize a more effective and efficient operating point along an optimum control path.
2. Description of the Prior Art
A prior art search directed to the subject matter of this application in the U.S. Patent and Trademark Office revealed the following U.S. Pat. Nos.:
2,983,111 PA1 3,174,298 PA1 3,248,896 PA1 3,355,906 PA1 3,522,711 PA1 3,633,073 PA1 3,780,532 PA1 4,142,838 PA1 4,156,578
The present invention is an improvement of U.S. Pat. No. 4,151,725 entitled "Control System for Regulating Large Capacity Machinery" which issued on May 1, 1979, and U.S. Pat. No. 4,282,719 also entitled "Control System for Regulating Large Capacity Rotating Machinery" which issued on Aug. 11, 1981. Both of these patents are assigned to the same assignee as this application. In the '725 Patent, the compressor head value was inferred from the difference from the temperature of the saturated refrigerant vapor leaving the evaporator and the temperature of liquid refrigerant leaving the condenser. This temperature difference was utilized with the control system to regulate both the means for adjusting the position of the guide vanes and the means for driving the compressor. However, the assumed linearity of the function between the isentropic compressor head parameter .OMEGA. and the difference between the condensing and evaporating refrigerant temperatures did not appear to be entirely accurate over the system operation at light loads and at low heads.
In the '719 Patent, there was provided an improvement over earlier U.S. Pat. No. 4,151,725 by recognizing that a more accurate indication of a compressor head at light loads and low heads could be determined by a signal which is a function of the expression (P.sub.cd -P.sub.ev )/P.sub.ev . The numerator of this ratio is derived from the difference between the condensor absolute pressure P.sub.cd and the evaporator absolute pressure P.sub.ev. The denominator of this ratio signal is derived from the evaporator absolute pressure P.sub.ev.
The present invention uses the inlet guide vanes position signal referred to as PRV or pre-rotational vanes signal taken from the electrical signal on line 62 of the '725 Patent and the absolute pressure signals in the condenser and in the evaporator taken from the transducers 110, 112 on respective lines 111, 113 of the '719 Patent. The substantial improvement of the present invention depends in part upon the addition of sensing a suction flow signal. With the continual measurements of the PRV signal, compressor head signal, and suction flow signal, the present invention includes a microprocessor responsive to these measurements to calculate an initial operating point. If this operating point is determined to be sufficiently away from a surge surface as generated from the microprocessor containing pre-stored surge surface equations, the microprocessor will cause the operating point to move to a new one of a lower compressor speed and a more open PRV position.
In another embodiment of the present invention, a compressor head signal is replaced with an actual motor speed signal of the compressor. Thus, the microprocessor uses the continual measurements of the PRV signal, motor speed signal and suction flow signal to calculate the initial operating point as well as determining whether there is a more optimum operating point of a lower compressor speed and a more open vane position as was previously described above.